This invention relates to multi-electrode welding of metallic workpieces, especially those made of aluminum, and, in particular, the invention relates to the reduction of so-called "arc blow" obtained when a workpiece is welded with a multi-electrode welding gun.
It is already known that welding speeds for arc welding seams and the like can be increased by increasing the heat transfer rate to the weld plate. One method of accomplishing this is to increase the arc current, but it has been found that when the current reaches about 200 to 300 amperes in normal operation, the welding speed cannot be further increased significantly by producing further increases in the arc current. The reason for this is believed to be that, instead of the heat being concentrated around the arc axis, it is instead distributed over a wider area. Furthermore, detrimental effects may also be produced by arc "pumping" at high currents, thus leading to undercut and weakened welds.
It is also known that, instead of increasing the arc current, the welding speeds can be increased by providing several welding electrodes arranged in a row in substantially equally spaced parallel relation to one another with the arc-producing ends of the electrodes being approximately equally spaced from the weld plate and aligned with the seam to be welded. Each of the electrodes provides a separate arc and the electrodes are all moved as a unit in a direction parallel to the seam to be welded. The heat from each of the electrodes adds together without substantial dissipation, so that the maximum welding speed is found to be much greater than the maximum obtained by the use of a single electrode, without any undesirable effects, such as undercutting. It is not usually necessary to provide more than four electrodes, although more than four may be used, and in some cases, only two or three may be required.
Canadian Pat. No. 749,527, issued on Dec. 27, 1966, to Union Carbide Corporation (corresponding to U.S. Pat. No. 3,242,309 issued Mar. 22, 1966), describes a system in which a welding gun is provided with a plurality of electrodes for providing relatively short welding arcs between the ends of the electrodes and the workpiece. During use, the arcs are shielded from the atmosphere by a protective inert gas and the electrodes are all moved as a unit in a direction parallel to the seam to be welded. The maximum value of the welding speed is disclosed as being equal to the number of electrodes multiplied by the normal welding speed of one such electrode.
It is well known, however, that a problem, known as "arc blow," is encountered when two or more electrodes are located close together in order to produce a plurality of arcs. Arc blow is produced by magnetic interaction between the arcs and is the tendency of the arcs to be attracted towards each other. When two arcs are used, the arcs are attracted to each other and thus deviate inwardly from the desired parallel paths. When more than two arcs are used, the leading arc tends to bend rearwardly and the trailing arc tends to bend forwardly, but the other arcs remain substantially unaffected. Arc blow reduces the effectiveness of the welding gun and thus reduces the maximum welding speed. In the past, a compromise situation has had to be found because, in order to minimize the dissipation of heat during welding and thus to maximize the welding speeds, the electrodes should preferably be located as close together as possible. However, as the electrode spacing is reduced, the arc blow effect increases.
The relationship between the spacing of the various electrodes is discussed in Canadian Pat. No. 749,527, and it is disclosed that the magnetic interaction between the arcs depends upon the number of arcs, the arc current, the arc length and the electrode spacing. It is suggested in the patent that the effect of arc blow can be minimized in the case of three or more electrodes each carrying 100 amperes or more if the spacing is 1/4 inch or more for short arc lengths of 1/32 to 1/16 of an inch, and half an inch or more for an arc length of 1/8 inch. For current levels of from 10 to 100 amperes/electrode an arc length below 1/32 of an inch, the patent discloses that the spacing between the arcs should be between 1/8 to 3/8 of an inch. Thus, in general, the arc length should be less than 1/8 inch and the electrode spacing should be no greater than one inch, center to center.
Although the above Canadian patent has suggested a set of conditions for minimizing the disadvantageous effects of arc blow, it would be better to provide alternative means for minimizing or eliminating the magnetic interaction between the electrodes in order to remove the above-mentioned constraints upon the electrode spacing and the arc length, etc., thus producing greater flexibility.
Furthermore, it has been found that the conditions for minimizing arc blow suggested in the above Canadian patent are not effective when the workpiece to be welded is aluminum or an aluminum alloy. Until now, essentially all of the commercial use of multi-electrode welding has been with stainless steel and therefore the problems encountered with the welding of aluminum products have not been fully appreciated. The reason why aluminum products should react differently from stainless steel is not absolutely clear, but it is thought that effective arc lengths may be greater than anticipated when aluminum products are welded, for the following reasons.
The arc lengths used when welding ferrous metals, and particularly stainless steel, are extremely short and the true arc length (measured from the arc-producing end of the electrode to the surface of the weld pool) does not differ greatly from the electrode-to-work distance (measured from the arc-producing end of the electrode to the surface of the work in the absence of an arc). Aluminum, however, is believed to behave very differently as the weld pool can be well below the surface of the parent metal. Thus the true arc length can be much greater than the electrode-to-work distance and this longer arc is more susceptible to arc blow.
An alternative method of controlling arc blow is thus required. One known method of controlling an arc is employed by Cyclomatic Industries Inc. of San Diego, Calif., U.S.A., in its electromagnetic probe system. In this system, a magnetic field is generated by an external power source in the region of an arc produced by a single-electrode welding gun. The magnetic field can be oscillated so that motion of the arc can be produced either parallel to or perpendicularly to the direction of movement of the welding gun. Alternatively, "wandering" of the arc can be eliminated by providing a strong stable magnetic field. However, the use of electromagnetic probes is not suitable to control arc blow in multi-electrode guns because of the complexity and high cost and because the electromagnetic fields used to correct the arc blow of the outermost arcs of a multi-electrode gun would interfere with the closely adjacent intermediate arcs. The electromagnetic probe system is thus only economical and effective when used with single electrode welding guns.